Eye movements and certain complex visual functions are influenced by neurobehavioural disorders such as (but not limited to) Parkinson's disease (PD), Alzheimer's disease (AD), fetal alcohol spectrum disorder (FASD), and attention deficit hyperactivity disorder (ADHD). Symptoms of Parkinson's disease include motor impairments, such as muscle rigidity and slowness of movement, impaired ability to suppress automatic behavioral responses, and difficulty in initiating voluntary actions, such as saccades. Saccades are rapid, ballistic eye-movements that bring the part of the retina with the highest acuity (the fovea) onto a target of interest, and are the most common type of eye-movement in human life with hundreds of thousands of saccades initiated per day. Individuals with FASD present with a wide range of cognitive impairments that include deficits in spatial working memory, planning, response inhibition and the ability to think abstractly and flexibly. ADHD is a highly prevalent neurobehavioural disorder characterized by inattention, hyperactivity, and impulsivity that may have an incidence as high as 5-10% in children and 2-4% in adults worldwide. Previous studies have explored these disorders and their relationship to top-down (goal oriented) and bottom-up (stimulus driven) attention by utilizing different visual and cognitive tasks. The tasks require participants to be restricted to performing specific behaviours that are varied across experiments.
Because of the dysfunction in voluntary control and response inhibition across these disorders, it is assumed that they are more governed by “stimulus-driven”, rather than “goal-driven” processes, and as such may be more governed by stimuli that are highly ‘salient’, i.e., capable of capturing attention. This can occur in a variety of neurobehavioral disorders, including schizophrenia, Alzheimer's disease, Huntington's disease (HD), autism, Tourette syndrome (TS) and progressive supranuclear palsy (PSP). However, the nature of the task can dictate whether these disorders surface or not, and this often leads to contradictory results. Each of these disorders also show that processing time of goal-driven behaviour seems to be slower; however, different conditions across tasks can lead to different measures of attentional deficits. Thus, while previous studies are valuable in dissecting particular impairments in attentional allocation, the often contradictory results show how difficult it is to use one parameter to identify underlying behavioural differences between diseases.